Self-cleaning expanded polytetrafluoroethylene-based hybrid membrane for water filtration

Membrane surface fouling is a key problem for water filtration. Compositing photocatalytic substances with a base membrane is a widely used strategy, but most of the membrane will be decomposed by photocatalysis. Herein, expanded polytetrafluoroethylene (ePTFE) with extremely stable chemical properties is grafted with polyacrylic acid (PAA) and then modified with titanium dioxide (TiO2) to realize a self-cleaning TiO2–PAA–ePTFE filtration membrane. It can recover its flux under UV irradiation after fouling. With 20 rounds of self-cleaning, the membrane microstructure still remains intact. Moreover, in addition to retaining bovine serum albumin, TiO2 particles on the membrane surface are capable of absorbing small organic pollutants and degrading them. Thus, this membrane is potentially used as an anti-fouling membrane for water filtration.


Introduction
Membrane separation has been applied for water treatment worldwide because it is characterized as economic, 1 energysaving, 2 easy to operate and high efficiency. 3,4 Over the past decades, a series of membranes made of polyvinylidene uoride, 5 polyvinyl chloride, 6 polysulfone, 7 polystyrene, 8 polyacrylonitrile 9 and polypropylene have been developed to meet different ltration requirements. However, membrane fouling is still a key challenge in practice. 10 Membrane fouling is generally attributed to organic foulants. Thus, a massive amount of work has focused on self-cleaning membranes through integrating photocatalysts. For example, Ayyaru et al. 11 prepared PES-STiO 2 membranes by a phase inversion method with the membrane ux of 502 to 802 L m À2 h À1 ; the recovery of water ux increased from 63% to 75.7 to 96.5%. Méricq J. et al. 12 prepared PVDF-TiO 2 lm by a phase conversion method, which reduced the contact angle of the original lm from 80 to 64 . Aer the membrane is polluted by BSA solution, the membrane performance can be completely restored by UV irradiation and water cleaning. Bojarska M. et al. 13 prepared PP/ plasma/ZnO lm by a chemical bath deposition method, and studied the effect of the lm on the degradation of methylene blue at different pH values. It was found that the best degradation effect was up to 90%. However, the photocatalytic function also brings about a key challenge for the membrane itself, because it can not only degrade the organic foulants but also attack the polymer structure of the membrane. Therefore, exploring a new catalysis resistant lm material is urgent for self-cleaning membranes. 14 Expanded polytetrauoroethylene (ePTFE) is a kind of porous material with millions of pores per square centimeter that made by special biaxial stretching process. 15 It inherits all the advantages of polytetrauoroethylene with excellent stability and low surface energy and is a promising lm material with catalysis resistant. However, its hydrophobic surface restricting the application in water treatment elds because the pores are not permeable to water. Several surface modication methods have been developed to address this problem, such as sodium-naphthalene treatment 16 high energy ray activation, 17 low temperature plasma-treatment. 18,19 Among these methods, the last one is the most promising strategy for the modication of water treatment ePTFE membranes because it has scarcely no damage to the microstructure of the membrane.
In this work, the photocatalysts TiO 2 particles were immobilized on ePTFE lm through a facile two-step method, i.e., surface graing of polyacrylic acid (AA) and hydrolysis of tetrabutyl titanate. The resultant membrane can degrade the fouling organics and maintain the high ux for tens cycles of ltration under UV irradiation. And the microstructure of the lm still remained intact. Moreover, the large specic surface area of the TiO 2 particles enable to absorb organics that smaller than membrane pore size and degrading them.
(SDBS, 98%), tetrabutyl titanate (TTB, 98%), absolute ethanol and acetone were purchased from Sinopharm Chemical Reagent Co., Ltd. Ultrapure water was applied in all experiment. All chemicals were analytical grade and used directly without any purication.

Membrane modication
The ePTFE lm was cut into a circle with a diameter of approximately 5.0 cm and rinsed with ethanol before use. Firstly, the ePTFE lm was treated by Ar-plasma at a power of 100 W for 4 min to form the radicals on the surface. The ePTFE lm was subsequently exposed to air for 120 min and used in subsequent surface modication experiments.
Plasma treated ePTFE lm was rst stuck inside a glass mold with size of 0.2 Â 10 Â 10 cm (thickness Â width Â height). The 20 wt% AA solution with sodium dodecyl benzenesulfonate (1.6 Â 10 À3 mol L À1 ) was then transferred to a glass mold, and nitrogen was applied in the solution of the glass mold for 5 min to remove dissolved oxygen. The glass mold was heated at 60 C for 180 min. 20 Aer modication experiment, the modied PTFE membranes were rinsed many times with ultra-pure water and dry it.
Modied PTFE lm was put into a mixed solution of ethylene glycol (10 ml) and tetrabutyl titanate (0.1 ml). This solution was stirred for 8 h at a vacuum glove box. Subsequently, solution of ethanol (0.25 mL) and 98% H 2 SO 4 (0.45 mL) were added to the above solution. Finally, 20 mL acetone was added and this mixture was transferred to a Teon-lined autoclave and maintained at 180 C for 6 h. Aer the experiment, the ePTFE lm was washed repeatedly with isopropanol and ammonia solution.

Membrane characterization
ATR-FTIR (Thermo Fisher Nicolet iS10, USA) was used to verify the modication of the membranes in the range of 4000-400 cm À1 . X-ray photoelectron spectroscopy (XPS, Bruker D8 advance, Germany) analysis was carried out to analyze the elemental composition of the membranes. The morphologies and microstructures (SEM, Thermo Scientic Apreo 2C, USA) of membranes are characterized by emission scanning electron microscopy. Energy dispersive spectrometer (EDS, OXFORD ULTIM Max65, Britain) was used to analyze the distribution of various elements on the lm. Atomic force microscope (AFM, Dimension ICON, USA) was used to measure the lm surface roughness. Automatic specic surface area and pore size analyzer (American micrometrics ASAP 2460) was used to measure the specic surface area of titanium dioxide. High performance automatic mercury porosimeter (micrometrics 9600, USA) was used to measure membrane pore size. Precision electronic universal material testing machine (Shimadzu AGX-V, JP) was used to analyze the mechanical properties of the lm. Synchronous thermal analyzer (Mettler Toledo TGA/DSC 3 + , CH) was used to analyze the thermal properties of the lms.

Contact angle and surface energy test
The water contact angle of each membrane was measured by a contact angle goniometer (optical angle meter and interface tensiometer, USA). For each measurement, 2 mL drops were formed using ultra-pure water and the subsequently reported contact angles were taken as an average of at least three measurements. The Owens-Wendt-Rabel-Kaelble solid surface free energy estimation method in the test angle measuring instrument is selected to calculate the surface energy of three different membranes, in which ultrapure water and diiodomethane are used as the test liquid.

Membrane ux test
The membrane ux of pure water was measured by dead end ltration experiment. The membrane in circle shape with a diameter of 4.6 cm was put it into a 50 ml ultraltration cup, and measured the membrane ux at 0.1 MPa aer 30 minutes of pre operation at 0.15 MPa. The membrane ux is calculated according to eqn (1).
where V (L) is the permeate volume, A (m 2 ) is the membrane ltration area, and Dt (h) is the permeate time.

Study on leaching and membrane stability
Leaching study. The prepared lms were immersed in 10 ml ultra-pure water under the irradiation of LED ultraviolet lamp with the distance of 50 cm away from the lm sample, in which the wavelength of ultraviolet lamp was 365 nm and the power was 60 W. Then the lms were taken out at different times (up to 120 hours) and placed in the photocatalytic device. 50 ml ultrapure water was circularly ltered under the irradiation of LED ultraviolet lamp to determine the titanium dioxide ltered from the membrane during the ltration process. The ltration pressure was 0.1 MPa. 1 ml of ltrate at different times (up to 8 hours) was taken out and digested with hydrouoric acid for titanium measurement using ICP-OES (PerkinElmer optima 8000 US).
Membrane stability experiment. The original ePTFE membranes were exposed to ultraviolet lamp for different times (up to 120 h) and adopted to membrane ux test and infrared test to investigate the anti-UV stability of the ePTFE base lm. The mechanical stability was studied by mechanical test of the TiO 2 -PAA-ePTFE lm aer 20 cycles of ltration under UV condition, and the lm without UV irradiation was also tested as control.

Photocatalytic degradation of organic wastewater
Rhodamine B (RB), tetracycline hydrochloride (TC) and bovine serum protein (BSA) were used as representatives of organic wastewater to evaluate the photodegradation ability of TiO 2 -PAA-PTFE. Fix the membrane on the bottom of the quartz beaker with an iron ring, add RB (10 ml, 5 ppm)/TC (10 ml, 15 ppm)/BSA (15 ml, 10 ppm), and then place the quartz beaker in the photocatalytic instrument. Keep it in the dark for 30 min to make organic wastewater and membrane reach adsorption equilibrium, then turn on the photocatalytic instrument, take out 2 ml every 30 min and detect it with UV-vis spectrophotometer, and take back to the quartz beaker aer testing. The protein content of bovine serum was measured by Coomassie brilliant blue method. 21

Synergistic effect of catalysis and adsorption and cyclic test
Rhodamine B (RB) and tetracycline hydrochloride (TC) were used as small molecule organic wastewater to test the synergistic effect of catalysis and adsorption of TiO 2 -PAA-ePTFE membrane. The instrument is composed of 50 ml ultraltration cup and UV-LED lamp band, which is circularly ltered with Rb (3 ppm, 20 ml)/TC (15 ppm, 20 ml). Under 0.01 MPa, 20 C and UV-LED lamp, 2 ml ltrate is taken every 30 minutes for detection and put back. Cycle test for a total of 2 hours.
The circulation test is the same as the above operation. Aer each test, the membrane needs to be cleaned with ultrapure water and ethanol, and then repeat the next test. The length of each experiment was 3 hours, and a total of 5 experiments were carried out.

Synergistic effect of catalysis and ltration and cyclic test
The photocatalytic ltration device is composed of a 50 ml ultraltration cup and a UV lamp with power of 60 W and wavelength of 365 nm. Bovine serum albumin (BSA) was used as a macromolecular organic wastewater to test the synergistic effect of catalysis and ltration of the TiO 2 -PAA-ePTFE membrane. Continuously pour 1 g L À1 BSA into the ultraltration cup and lter at 20 C, 0.1 MPa until the membrane ux drops below 20%. Stop the ltration. Remove the lm and operate: (1) Aer 25 minutes of LED UV irradiation, the membrane was immersed in ethanol for ultrasonic cleaning for 1 minute, and nally cleaned with ultrapure water; (2) the membrane was cleaned with ultrapure water, then immersed in ethanol for ultrasonic cleaning for 1 minute, and nally cleaned with ultrapure water. Record the time every 50 ml of ltrate collected during ltration. Aer each cycle test, perform operation (1) and then start the next cycle test.

Results and discussion
ePTFE lm has millions of pores per square centimeter, 15 which is an ideal lter material. However, it is difficult to apply to the eld of water ltration because of its super-hydrophobic nature. 22 Herein, the ePTFE lm is composited with hydrophilic titanium dioxide (TiO 2 ) particles to improve its water permeability. As shown in Scheme 1, ePTFE lm was modied with TiO 2 particles through a two-step method. In step 1, polyacrylic acid (PAA) was graed onto ePTFE lm through lowtemperature plasma technology. The Ar plasma-pretreated ePTFE lms were exposed to air to form peroxide and hydroperoxide groups. Subsequently, these groups were decomposed into free radicals by heat and initiated the polymerization of acrylic acid (AA) monomers. 23 In step 2, TiO 2 particles were grown in situ on ePTFE lm through solvothermal method with the advantage of the graed PAA chains. In this step, the hydrolysis of tetrabutyl titanate (TiO(OH) 2 ) was connected with PAA by ionic coordination and hydrogen bonding, and then grown into solid particles. 24,25 As a result, the ePTFE lm loaded with TiO 2 was prepared and is abbreviated as TiO 2 -PAA-ePTFE in the following.
To verify the whole preparation process, the surface properties of the lms in each preparing step were characterized. As Fig. 1a shows, infrared absorption at 1714 cm À1 appears in the spectrum of ePTFE lm graed with PAA (PAA-ePTFE), which is related to the stretching vibration of carbonyl groups, indicating that PAA chains have been successfully graed onto ePTFE surface. 26,27 Aer compositing with TiO 2 , the vibration and stretching of Ti-O bond appeared at 800 to 500 cm À1 in the spectrum of TiO 2 -PAA-ePTFE. 25,28 In addition, a wide peak around 3300 cm À1 caused by Ti-OH interaction and surface absorbed water has also been observed. XRD was used to analysed the crystal structure of ePTFE lms. Just as seen in Fig. 1b, the ePTFE characteristic peaks at 2q ¼ 17.99 were (1, 0, 0) crystal plan. The characteristic peak intensities of the ePTFE, PAA-ePTFE, and TiO 2 -PAA-ePTFE lms decreased sequentially, which resulted from the destruction of the crystal forms on the surface of the lms aer graing of PAA and TiO 2 . The chemical composition of ePTFE lms surface was determined via XPS analysis. Fig. 1c shows that the original ePTFE lm is mainly consisted of C and F elements which only has C1s and F1s peaks. Aer graing PAA, O1s (534 eV) peak appeared, indicating that the carboxyl groups have been modied to the Scheme 1 Schematic diagram of TiO 2 -PAA-ePTFE film preparation process. surface of ePTFE membrane. In the high-resolution Ti2p XPS spectra of TiO 2 -PAA-ePTFE lms (Fig. 1c), double peaks appeared at Ti2p 1/2 (464.4 eV) and Ti2p 3/2 (458.6 eV), which accords with the Ti XPS in TiO 2 reported in literature. 29 In particular, the F1s peak intensity decreases sharply due to the increasing of O1s and Ti2p, indicating that TiO 2 has been successfully graed on the surface of ePTFE lm, which agreed with the results of FT-IR results. Further, the atomic percentage values of all the lms are shown in Fig. 1d. It can be seen that F atom decreases from 90.5% to 28.8%, O atom rises from 0 to 18.3%, and Ti atom accounts for 6.9% of the lm surface.
The surface morphology of modied ePTFE lms was imaged using SEM. Fig. 2a shows that the ePTFE lm has a microporous structure composed of a large number of interconnected bers. Aer graing with PAA, the ber structure of ePTFE stays integrity and bulges appeared on its surface (Fig. 2b). The PAA-ePTFE has signicantly increased roughness over the virgin ePTFE lm (Fig. 2a). As shown in Fig. 2c, a large amount of hedgehog shaped TiO 2 particles are observed on the surface of ePTFE, and covering the brous structure. The EDS scan images of the TiO 2 -PAA-PTFE lms are shown in Fig. 2f. The results show that the Ti and O elements are uniformly distributed on the surface of the TiO 2 -PAA-ePTFE lms. This indicates that TiO 2 particles have been successfully prepared on ePTFE lm. Fig. 2d shows a clear view of the TiO 2 -PAA-ePTFE lm. The brous network is still visible and well combined with TiO 2 particles. This is favorable for the water permeability of the lm. As Fig. 2g shows, the original lm has the characteristics of super hydrophobicity and ultra-low surface energy with water contact angle of $140 . With the modication of PAA and TiO 2 , the contact angles are obviously decrease to 76 AE 6 and 31 AE 8 , respectively, and the surface energy increased signicantly from 3.2 mN m À1 to 89.1 mN m À1 . In addition, the hedgehog shaped TiO 2 shows many radial shaped needle-like antennae on the surface of the crystal nucleus (Fig. 2e), with increased adsorption capacity (specic surface area: 234.5 m 2 g À1 ). 30,31 Fig . 2i shows the membrane ux of water for lms. In respected to the original ePTFE lm, the membrane ux of the PAA-ePTFE and TiO 2 -PAA-ePTFE increased by nearly two orders of magnitude, which are 1835 AE 10 L m À2 h À1 and 1627 AE 84 L m À2 h À1 , respectively. It can also be seen intuitively from the insert photos that the ePTFE lm is completely opaque in water, and it becomes transparent aer graing with PAA. That is, water can easily penetrate through the modied PAA-ePTFE lm. While, the opacity of the water-permeable TiO 2 -PAA-ePTFE is caused by a layer of white TiO 2 particles formed on its surface. Compared with the original ePTFE lm, the modied ePTFE shows a larger tensile strength (Fig. S1a †) and retains a nearly constant thermal stability (Fig. S1b †), which will be benecial to ltration under high pressure (Fig. S1 †).
The surface topographies and roughness of the ePTFE lms were observed with AFM (Fig. 3). It can be clearly seen that the AFM image of virgin ePTFE lm has a microporous structure (Fig. 3a). Aer graing with PAA (Fig. 3b), the brous structure disappeared and the roughness become much bigger. Then as loaded with TiO 2 particles, the TiO 2 -PAA-ePTFE shows the roughest surface over the previous two groups (Fig. 3c and e) and the pores become smallest (Fig. 3f). From the line scanning path (Fig. 3d), it can be seen that the width of the bulge on the TiO 2 -PAA-ePTFE surface is about 1 mm, which is consistent with the particle size of TiO 2 observed in SEM.
Ultraltration (UF) membranes are generally used for ltering substances with size of 100 to 1000 nm. During the application, most UF membranes will face the problem of channel blockage. Herein, the TiO 2 -PAA-ePTFE lms loaded with TiO 2 particles aim to disclose this problem. The change of membrane ux with time was measured by bovine serum albumin (BSA) to study the clogging and self-cleaning of the TiO 2 -PAA-ePTFE lm. BSA will blocking the membrane channels and resulting in the decrease of membrane ux. Then UV light is used to drive the self-cleaning function of the lm and  restore its ux. The experimental results are shown in Fig. 4a, when continuously ltering 1 g L À1 BSA solution, the membrane ux decreased continuously to 20% of its original value aer 70 min. At this time, a 365 nm UV light is adopted to the lm, and triggering the TiO 2 to catalyze the BSA decomposition of the blocked lm and wash it away. Then, the membrane ux completely recovers to the original level. However, if only pure water is used instead of UV irradiation, the membrane ux can only recover to 40%. And If UV irradiation is re-enabled, the ux can return to the original level aer 25 minutes (Fig. S2 †). Fig. 4b shows the change of membrane ux with the use times. Surprisingly, the membrane ux can recover to 100% aer 10 cycles. Even for 20 times, the selfcleaning function can guarantee $90% of membrane ux. The falling off of titanium dioxide will cause secondary pollution of the ltrate, so it is necessary to study the leaching behavior. In the 120 h immersion leaching experiment (Fig. 4c), the leaching rate of titanium dioxide is less than 0.04% (percentage of total titanium dioxide), and in the ltration leaching experiment (Fig. 4d), aer the photocatalytic membrane was ltered under UV irradiation for 480 min, the leaching rate of titanium dioxide was less than 0.04%. These results show that the coordination between titanium dioxide and ePTFE is stable and has long-term application prospects. The stability of photocatalytic membrane in ultraviolet light is also one of the very important properties. Aer 120 h of UV irradiation, the infrared spectrum of ePTFE membranes has seldom change in respected to the original one (Fig. 4e). The intensity of the vibration absorption peaks of CF 2 in 1046.5 cm À1 and 1202.8 cm À1 remains constant. The ux change rate of ePTFE membrane is less than 0.1% aer 120 h of UV irradiation (Fig. 4f). At the same time, aer 20 cycles, the stress-strain curve of TiO 2 -PAA-ePTFE membrane still maintains its original properties (Fig. 4g). These results indicating that ePTFE has excellent UV resistance. The micromorphology of the membrane aer UV irradiation has been further observed by SEM. In order to observe the porous structure clearly, the TiO 2 particles on membrane surface were corroded by concentrated sulfuric acid. As shown in Fig. 4h and i, the membrane used for 20 cycles has the similar microstructure to the freshly prepared one, indicating the PTFE network is stable to the UV irradiation. This photocatalytic induced self-cleaning will provide a new design idea for prolonging the service life of the ultraltration membrane.
Generally, lter membrane only retains the substances with size larger than its pore size. But surprisingly, the TiO 2 -PAA-ePTFE lm can absorb and decompose small molecular organics. Fig. 5a shows the molecular adsorption capacity of ePTFE, PAA-ePTFE and TiO 2 -PAA-ePTFE lms in respect to TC (10 ml, 15 ppm) and RB (10 ml, 5 ppm), respectively. Among them, ePTFE can barely absorb these molecules because of their ultra-low specic surface energy. When modied with PAA and TiO 2 , the adsorption capacity of the modied lm to organic molecules is signicantly improved. Especially for TiO 2 -PAA-ePTFE lm, its adsorption capacity for TC and RB are 103.7 and 46.1 mg mg À1 , respectively. Fig. 5b shows the real appearance of  the ePTFE, PAA-ePTFE and TiO 2 -PAA-ePTFE lms aer absorbing of RB and TC. The results are consistent with the experimental data, and the color change of TiO 2 -PAA-ePTFE lm is the most obvious. Fig. 5c shows the photocatalytic curves of TiO 2 -PAA-ePTFE for RB and TC. For both of them, the residual concentration ratios decrease rapidly in the initial 30 min without UV irradiation. This is due to the adsorption of TiO 2 to the substrates, which leads to the reduction of the residual in solution. Then, under UV light, photocatalytic effect of TiO 2 leads to the further reduction of residual concentration ratios to 98% and 83% for RB and TC, respectively. The nal treatment volume of RB is 19.9 ml (the initial volume is 20.0 ml). In addition, the antibacterial activity of the TiO 2 -PAA-ePTFE membrane has also been studied. Surprisingly, it was found that its antibacterial activity was as high as 98.2% (Table S1 †). As Fig. 5d shows, aer UV irradiation, the dyes absorbed on the lms have been totally clean out. Thus, the TiO 2 -PAA-ePTFE lm has power for degradation of small molecular organics.
Surface adsorption is benecial to catalytic degradation, while continuous degradation provides power for sustained adsorption. Fig. 5e shows the diagrams of the synergistic effects of ltration and catalysis on small molecular organics. In the ltration process of TiO 2 -PAA-ePTFE membrane, a large number of RB molecules are adsorbed on TiO 2 due to the adsorption of TiO 2 -PAA-ePTFE, which reduces the RB content in the ltered water. Under UV irradiation, the adsorbed RB decomposes, and TiO 2 continues to adsorb RB in water, which signicantly reduces the RB content in ltered water.
In the application of wastewater ltration, it is very important to evaluate the reusability and stability of the photocatalytic performance of the membrane. As shown in Fig. 5f, aer ve cycle experiments, it was observed that the degradation rate of RB by TiO 2 -PAA-ePTFE membrane was still about 90%. The lm has excellent reusability and stable performance. The reported parameters that relate to recent hybrid membrane development and features of the membranes are summarized in Table 1. The membrane in this work has the outstanding performance in FRR, reusability and pure water permeance.

Conclusions
In this work, ePTFE lm is graed with PAA chains so that TiO 2 can immobilized on lm surface in situ. With the composition of hydrophilic TiO 2 particles, membrane ux of the obtained TiO 2 -PAA-ePTFE membrane increases signicantly from 150 to 1627 L m À2 h À1 in respect to the pristine ePTFE lm. On one hand, the photocatalytic TiO 2 particles endow the membrane with self-cleaning performance, that is, aer surface fouling, the membrane ux can be restored to the original value under UV irradiation. On the other hand, the ePTFE lm ensures the stable physical-chemical properties of the membrane. Aer 20 rounds of UV irradiation, its porous structure still remains intact. Moreover, the hedgehog shaped TiO 2 particles with high specic surface area are helpful for the membrane to absorb and degrade small organic pollutants such as RB and TC. Overall, the resultant TiO 2 -PAA-ePTFE membrane can be potentially used as a self-cleaning water ltration membrane.

Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the nal version of the manuscript.

Conflicts of interest
There are no conicts to declare.